This application claims the priority of German application No. 197 38 513.3, filed Sep. 3, 1997, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to the use of a metal membrane foil consisting of a palladium alloy, especially one made of an alloy of palladium and a metal from Group VIII or IB, for separating hydrogen from a mixture of process gases.
Metal membrane foils for hydrogen separation can be used, as is known, for obtaining highly pure hydrogen from a gas mixture of whose components only the hydrogen is able to diffuse through the membrane foil. An important area of application is methanol reformation systems for supplying fuel cell systems in motor vehicles with the necessary hydrogen. The product gas that results from the reformation process, in addition to the desired hydrogen, also contains carbon monoxide, carbon dioxide, and water vapor. Hydrogen separation from this product gas using a membrane foil is a known way of providing hydrogen for the fuel cells, with the entry of carbon monoxide in excessive concentrations into the fuel cells having to be prevented in particular, since this gas acts as a catalyst poison there. For other applications, including in the electronics industry, metal membrane foils for providing highly pure hydrogen, for example with a purity of more than 99.9999% are used.
Metal membrane foils for use in hydrogen separation differ from membrane foils made of other materials such as ceramics, glasses, and polymers by their high selectivity and temperature stability. The metal membrane foils used for hydrogen separation often consist of palladium or palladium alloys. Palladium is characterized by a high hydrogen storage capacity even at room temperature and low hydrogen pressures. However, pure palladium is not suitable for use as a membrane foil, since a xcex2 hydride phase is produced in a certain temperature range that can lead to embrittlement and hence to crack formation. Therefore, an alloy partner from Group VIII or IB is usually added to the palladium. In commercially available hydrogen separation units, rolled foils that are typically 50 xcexcm to 100 xcexcm thick are made, produced from such a palladium alloy.
The manufacture of very thin foils with thicknesses of less than 50 xcexcm by rolling is theoretically cumbersome and cost-intensive because of the various rolling and intermediate annealing steps. In addition, the rolled foils with the smallest thicknesses can only be obtained with a maximum width of about 150 mm. Therefore, the electronics industry has resorted in the meanwhile to galvanic production of foils from copper or nickel. In this way, foil thicknesses of 20 xcexcm or less can be obtained with foil widths of 1.5 m to 2 m. In this conventional galvanic manufacture, the copper or nickel is precipitated from.a galvanic bath onto a stainless steel roller that dips into the galvanic bath and rotates.
It is known from Offenlegungsschrift JP 1-222085 (A) to use multilayer metal foils made of Cuxe2x80x94Fe, Fexe2x80x94Ni, Cuxe2x80x94Ni, or Cuxe2x80x94Fexe2x80x94Ni for PC board traces and magnetic shielding and to produce them by conducting a stainless steel strip, following bright annealing, sequentially through electrolyte baths, each of which is suitable for deposition of one layer at a time of the multilayer structure of the resultant metal foil.
DE 452 788 describes a method for galvanic manufacture of metal plates of uniform layer thickness in electrolyte baths with different compositions, in which the deposition of the metal materials takes place on an endless belt which passes continuously through the electrolytic baths and consists, for example, of copper. The belt serves as the mother cathode and is guided by a corresponding system of rollers. It can dip alternately for example into a copper bath and a nickel bath in order to produce copper and nickel layers in any repeated alternate sequence of layers. The metal plates produced in this fashion are used for catalytic reactions and storage batteries.
Offenlegungsschrift JP 1-201429 (A) describes the manufacture of metal foils made of a Bexe2x80x94Cu alloy by plating Be on a foil made of copper or of a copper alloy, with heat treatment being performed in an inert gas or a reducing gas atmosphere after the plating process to produce alloy-forming thermal diffusion. The metal foil thus produced can be used for electrical switches and relays and as a material for electromagnetic shielding.
Offenlegungsschrift JP 1-104792 (A) describes the electrolytic manufacture of a metal foil from an Fexe2x80x94Ni alloy with a high nickel content, with galvanic deposition taking place on a cathode drum made of Ti, Ta, Nb, or a Taxe2x80x94Nb alloy with a defined surface roughness and special process parameters in order to achieve the high nickel content.
An object of the present invention is the advantageous use of a palladium-containing metal membrane foil in hydrogen separation, for example in methanol reformation systems for fuel-cell-operated motor vehicles.
The present invention achieves this object by providing a use with a metal membrane foil manufactured by the steps of preparing at least one galvanic bath of several different bath types, with each bath type containing an electrolyte solution suitable for the deposition of one of the metallic alloy partners, and forming a layer stack for metal membrane foil by successive alternate galvanic deposition of layers consisting of one of the metallic alloy partners contained in the various types of baths, on the circumferences of deposition rollers that dip into galvanic baths of the baths in question, is electrically wired as a cathode, and is set rotating.
The metal membrane foil used in this way is built up from alternate layers of the different alloy partners which are each deposited galvanically. For this purpose, the next layer is applied to the layer structure reached up to that point on the circumference of a deposition roller that is electrically charged as a cathode and caused to rotate.
With this procedure, metal membrane foils can be produced in smaller thicknesses at comparable expense than with a rolling technique, which is especially advantageous for foils that are to be used for permeation purposes, such as the present case of hydrogen separation. Halving the foil thickness results in a doubling of the permeation rate, so that the material requirements for a given required permeation rate are smaller by a factor of four.
Membrane foils so manufactured from a palladium alloy are especially suitable for separating hydrogen from the product gas of a methanol reformation reaction.
A further aspect of the present invention uses a metal membrane foil consisting of a homogeneous alloy. To manufacture the foil, the metal membrane foil built up from the alternate layers of the metals involved is subjected to a tempering process selected so that intermetallic diffusion takes place between the individual layers, so that the layers of the different metals join to form a homogeneous alloy. This method is especially suitable for producing a metal membrane foil that is built up from a palladium alloy composed of two metals.
According to another aspect of the present invention, a membrane foil is used in which at least some of the layers are deposited by the continuous process. To work the continuous process, a suitable number of successive galvanic baths containing various metals involved and each having corresponding rotating deposition rollers is provided, and the metal foil is guided successively over the successive deposition rollers, dipping into the respective galvanic baths to lay down the respective layers.